A long-standing problem with wireless communications (and particularly in the context of RF communications) is the need to maximize spectrum efficiency. Spectrum efficiency refers to the most efficient use of the limited bandwidth that is set aside for a given type of wireless communication service. For instance, in the United States, the FCC has set aside the portions of the frequency spectrum from 851 to 869 MHz and from 806 to 824 MHz for trunked private two-way radio communications (the first portion of the spectrum being reserved for transmissions by a base unit and the latter portion being reserved for transmissions by mobile units). The reserved portion of the spectrum is divided into pre-defined frequency channels. A wireless communication system must be designed to transmit only within its defined channel or channels and must not interfere with (generate appreciable transmissions on) other channels within the spectrum. Channels are typically 30 kHz, or 25 kHz, or in some cases 12.5 kHz, depending upon the portion of the spectrum and the applications for which that portion of the spectrum has been reserved.
Competing with the need to maintain transmissions within a narrowly defined channel is the desire to maximize the throughput or utilization of the channel--i.e. to transmit as many communication paths between, e.g. mobile units and a base unit, as possible. Every communication path requires a certain amount of available bandwidth to transmit information (be it data, voice signals, or some combination).
In a trunked frequency division multiplexing system (FDM), each communication path is assigned to the next available frequency channel. In this way, only one communication path can be maintained at any one time on any given channel. As will be discussed below, actually two channels are used for two-way radio communications. The first channel, known as the forward channel, is used by the base unit to transmit to a mobile unit. The mobile unit transmits back to the base unit on a paired channel, referred to as the reverse channel.
In a time division multiplexing system (TDM), each channel is logically divided into several time slots. In this way, more than one communication path can be established on each channel, although only one communication path is active (i.e. transmitting) on the channel at any given instant. As an example, assume a TDM system in which the channel, the reverse channel, has been logically divided into two time slots. A first communication path can be allocated to the first time slot. The mobile unit will transmit during the first time slot only. A second mobile unit (establishing a second communication path) will transmit on the same frequency channel, but only during the-second time slot.
The base unit transmits information back to the mobile units on the forward channel, which is also logically divided into two time slots. The base unit transmits information to the first mobile radio during the first time slot of the forward channel and transmits information to the second mobile radio during the second time slot. Although the mobile units receive the base unit transmissions across both time slots, each mobile unit is instructed only to accept those transmissions which occur during the time slot associated with that mobile unit and to ignore the transmissions associated with the other time slot. In this way, two communication paths can be maintained on the same channel pair as was required for a single communication path on an FDM system.
Advances in the art have led to the recognition that a frequency channel can be sub-divided into separate sub-channels. As an example, a 25 kHz channel can be divided into sub-channels with sufficient guard bands to minimize interference between the sub-channels. In the prior art, it has been recognized that TDM communications can be made more robust by broadcasting across the four separate sub-channels in parallel, rather than broadcasting one signal across the full frequency channel. In other words, rather than transmitting one 64 kb/s signal in each time slot, the signal is broken into four (4) sub-channel signals, each of which transmits at a rate of 16 kb/s. This method does not increase the capacity of the channel, but does minimize undesirable effects of radio transmission, such as multi-path propagation interference and time delay spread. Those effects introduce distortion into the transmission, thus making the communication less robust and less reliable.
As demand for wireless communication increases, spectrum allocation has become increasingly important. One approach the FCC has taken to extracting more channels from the limited spectrum is to narrow the defined bandwidth of the channels--a technique known as channel splitting. Prior art systems, which are designed and implemented to rely upon the full bandwidth of a channel, must be redesigned and substantially retrofitted in order to operate within more narrowly defined channels. The burden and expenses associated with this is not justified--meaning that the prior art systems become obsolete as channels are re-defined to make more efficient use of the spectrum. In other words, the prior art systems are not scalable.
Another shortcoming of prior art devices is their limited ability to transmit only one type of information format (e.g. voice only, data only, voice and data) on a given channel. Because the prior art systems require the full channel bandwidth to transmit and because the receiving device must be programmed to recognize the type of information that is being transmitted on that channel, normally only one type of information can be transmitted on the channel at any given time.
An additional shortcoming of the prior art is the need to have enough power to generate a sufficient signal to overcome the inevitable noise that is introduced to the signal during transmission. As mobile devices become increasingly smaller and lighter, and as consumers come to expect longer battery operating times, the need to minimize the amount of power required to transmit becomes increasingly important. As the given transmission power of a mobile device is spread across a bandwidth (e.g. a 25 kHz or 30 kHz channel), the signal to noise ratio is reduced.
These and other shortcomings of the prior art are overcome in the preferred embodiments of the present invention, as will be described in more detail below.